All RF amplifiers are nonlinear to some degree, and consequently generate intermodulation (IM) components if the input signal fluctuates in amplitude. In most applications of interest, there are strict limits on the level and frequency distribution of IM power. Examples include mobile and satellite links carrying either single channels with non-constant envelope or multiple channels in any modulation format. The latter is common in base station transmitters. Another example is a cable television (CATV) system, with either coax or fibre trunks.
One solution is to use an inefficient Class A amplifier, and back it off to an even more inefficient operating region where it is reasonably linear. The result is an expensive amplifier with high power consumption and high heat dissipation. An alternative is to employ a linearizer, that is, a device that largely corrects the nonlinear characteristic of less expensive and more efficient Class AB or Class B amplifiers. Linearization of Class C amplifiers is also possible, but is not particularly useful.
The present inventor's U.S. Pat. No. 5,049,832 presents a digital signal processor (DSP) based solution that provides precision signal manipulation, so that the combination of the amplifier and a predistorter is highly linear. A potential drawback of that approach is the computational limit of DSP integrated circuit chips, which, for currently available technology, effectively restricts its applications to a single mobile channel.
The alternative presented here is a feedforward linearizer. It works directly with RF signals in and out, and uses analog technology to manipulate them. Analog technology has its limitations, but narrow bandwidth is not one of them. The feedforward linearizer has the potential to linearize a base station amplifier over the entire 20 Mhz mobile band, an operating range inaccessible to the aforementioned prior art, thereby facilitating accommodation of multiple carriers in a single amplifier.
In simplified terms, a feedforward linearizer cascades a signal cancellation circuit and a distortion cancellation-circuit. The signal cancellation circuit has two branches, one of which contains the power amplifier whose output is to be linearized. In particular, the amplifier's output consists of an amplified version of an input signal, plus IM distortion. The other branch of the signal cancellation circuit contains circuitry characterized by a coefficient .alpha. (amplitude and phase) that can be adjusted to match the amplitude and phase shift of the amplifier, and a delay, also chosen to match the amplifier. If the match is perfect, the error signal obtained by subtracting the output of the two branches of the signal cancellation circuit equals the IM distortion. In the distortion cancellation circuit, an appropriately amplified and phase shifted version (coefficient .beta.) of the distortion is subtracted from the amplifier output, ideally leaving only the linearly amplified replica at the feedforward output.
The above-described feedforward configuration has been known and studied for over 30 years, but methods for automatic adjustment of the coefficients .alpha. and .beta. have only recently been proposed. Those coefficients, and other component tolerances, such as the time delay match in the upper and lower branches of the signal cancellation circuit, are critical to deep suppression of IM power, since the feedforward method is based on subtraction of nearly equal values. Adaptation of the coefficients is essential in an environment where the number of carriers and the operating temperature can both fluctuate, changing the amplifier characteristics with them.